In U.S. Pat. No. 3,708,296 issued Jan. 2, 1973 entitled Photopolymerization of Epoxy Monomers," and commonly assigned herewith, there are disclosed and claimed novel compositions comprising various epoxy materials and certain latent catalyst precursors therefor. U.S. Pat. Nos. 3,721,616 and 3,721,617 issued Mar. 20, 1973 as well as U.S. Pat. Nos. 3,711,390 and 3,711,391 issued Jan. 16, 1973 disclose similar compositions comprising gelation inhibitors as well. Such compositions are photosensitive and when exposed to an energy source such as actinic radiation, yield epoxy polymers which are receptive to ink and possess inherent toughness, abrasion resistance, adherence to metal surfaces, resistance to chemical attack, etc. and are thus valuable for many applications, particularly those involving formation of acid and alkali resist images for chemical milling, gravure images, offset plates, stencil making, etc.
Many of the polymerizable compositions proposed in the patents referred to hereinabove are essentially solventless wherein the catalyst can be directly dissolved into the liquid epoxide and use of a solvent is not a critical consideration in these cases. However, in a commercial operation where large quantities may be involved and where highly viscous materials may be involved, the dissolution process, in the absence of solvents, is time-consuming. Additionally, a considerable volume of air may become entrapped in the formulation during the dissolution and, where the system is pigmented, it is often difficult, if not impossible, to determine the point at which all of the catalyst has gone into solution. Thus, solvents are often necessary components of such polymerizable compositions.
In practice such photopolymerizable compositions have been utilized as a three-package system, e.g. the epoxide, diazonium catalyst, and solvent with or without a stabilizer have each been supplied separately and combined just prior to use. The three-package system has been necessary for two reasons: In some instances, the shelf-life of catalyzed epoxy formulation has not been great enough to allow for greatly extended periods of time between catalyzing the formulation and using it for the end purpose, and secondly, there has not been available a solvent which possessed adequate dissolving power for the diazonium catalyst yet was sufficiently inert thereto so that a catalyst solution could be prepared and stored for several months for subsequent use without significant loss of curing ability or introduction of gross amounts of color to the cured film. Acetonitrile has been found to be a potentially attractive material for use in such a two-package system because not only is it an excellent solvent for the diazonium catalysts, but in addition it gives greatly extended shelf-life to the catalyzed epoxy formulation. Unfortunately, acetonitrile has two major disadvantages: it is a highly toxic material and it is a very volatile material. Thus, because of the potential hazards involved in its use, it has not provided the solution to the problem. Accordingly, there is a need in the art for a solvent for diazonium catalysts which is substantially inert to the catalyst yet has adequate dissolving power therefor, is low in volatility, is low in toxicity and is readily available.